Earthquake

I

Introduction

Earthquake, shaking of the Earth’s surface caused by rapid movement of the Earth’s rocky outer layer. Earthquakes occur when energy stored within the Earth, usually in the form of strain in rocks, suddenly releases. This energy is transmitted to the surface of the Earth by earthquake waves. The study of earthquakes and the waves they create is called seismology (from the Greek seismos, “to shake”). Scientists who study earthquakes are called seismologists.

The destruction an earthquake causes depends on its magnitude and duration, or the amount of shaking that occurs. A structure’s design and the materials used in its construction also affect the amount of damage the structure incurs. Earthquakes vary from small, imperceptible shaking to large shocks felt over thousands of kilometers. Earthquakes can deform the ground, make buildings and other structures collapse, and create tsunamis (large sea waves). Lives may be lost in the resulting destruction.

Earthquakes, or seismic tremors, occur at a rate of several hundred per day around the world. A worldwide network of seismographs (machines that record movements of the Earth) detects about 1 million small earthquakes per year. Very large earthquakes, such as the 1964 Alaskan earthquake, which caused millions of dollars in damage, occur worldwide once every few years. Moderate earthquakes, such as the 1989 tremor in Loma Prieta, California, and the 1995 tremor in Kōbe, Japan, occur about 20 times a year. Moderate earthquakes also cause millions of dollars in damage and can harm many people.

In the last 500 years, several million people have been killed by earthquakes around the world, including over 240,000 in the 1976 T’ang-Shan, China, earthquake. Worldwide, earthquakes have also caused severe property and structural damage. Adequate precautions, such as education, emergency planning, and constructing stronger, more flexible, safely designed structures, can limit the loss of life and decrease the damage caused by earthquakes.

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II

Anatomy of an Earthquake

Seismologists examine the parts of an earthquake, such as what happens to the Earth’s surface during an earthquake, how the energy of an earthquake moves from inside the Earth to the surface, how this energy causes damage, and the slip of the fault that causes the earthquake. Faults are cracks in Earth’s crust where rocks on either side of the crack have moved. By studying the different parts and actions of earthquakes, seismologists learn more about their effects and how to predict and prepare for their ground shaking in order to reduce damage.

A

Focus and Epicenter

The point within the Earth along the rupturing geological fault where an earthquake originates is called the focus, or hypocenter. The point on the Earth’s surface directly above the focus is called the epicenter. Earthquake waves begin to radiate out from the focus and subsequently form along the fault rupture. If the focus is near the surface—between 0 and 70 km (0 and 40 mi) deep—shallow-focus earthquakes are produced. If it is intermediate or deep below the crust—between 70 and 700 km (40 and 400 mi) deep—a deep-focus earthquake will be produced. Shallow-focus earthquakes tend to be larger, and therefore more damaging, earthquakes. This is because they are closer to the surface where the rocks are stronger and build up more strain.

Seismologists know from observations that most earthquakes originate as shallow-focus earthquakes and most of them occur near plate boundaries—areas where the Earth’s crustal plates move against each other (see Plate Tectonics). Other earthquakes, including deep-focus earthquakes, can originate in subduction zones, where one tectonic plate subducts, or moves under another plate. See also Geology; Earth.

B

Faults

Stress in the Earth’s crust creates faults, resulting in earthquakes. The properties of an earthquake depend strongly on the type of fault slip, or movement along the fault, that causes the earthquake. Geologists categorize faults according to the direction of the fault slip. The surface between the two sides of a fault lies in a plane, and the direction of the plane is usually not vertical; rather it dips at an angle into the Earth. When the rock hanging over the dipping fault plane slips downward into the ground, the fault is called a normal fault. When the hanging wall slips upward in relation to the footwall, the fault is called a reverse fault. Both normal and reverse faults produce vertical displacements, or the upward movement of one side of the fault above the other side, that appear at the surface as fault scarps. Strike-slip faults are another type of fault that produce horizontal displacements, or the side by side sliding movement of the fault, such as seen along the San Andreas fault in California. Strike-slip faults are usually found along boundaries between two plates that are sliding past each other.

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